1. Field of the Invention
The present invention relates to a covering machine which is arranged to twist covering yarns such as polyamide and nylon yarns around elastic yarns of polyurethane material.
2. Description of Related Art
A basic structure of a covering machine is known in a publication such as Japanese Published Examined Patent Application No. 56-20372. The covering machine is constructed of a multiplicity of spindle elements provided with a multiplicity of core yarn bobbins, spindles, take-up drums and the like.
The spindle is arranged to support a covering yarn bobbin and to rotate by itself, and a prior art spindle driving device is designed as shown in FIG. 5.
To be more precise, the spindle driving device has been designed in such a manner that a multiplicity of spindles 31 are disposed on both sides of the covering machine and an endless belt 34 is stretched between a drive pulley 32 mounted at one end of the covering machine and a driven pulley 33 arranged at another end of the machine. A multiplicity of tension rollers 35 are provided for pressing the endless belt 34 to closely contact each one of the spindles 31. The drive pulley 32 is driven by an electric motor (not shown) to rotate the spindles 31 at a high rate of revolution which corresponds to the rotative movement of the endless belt 34.
However, the spindle driving device used in the conventional covering machines suffers from several problems.
For example, it is difficult to produce various kinds of covering yarns through use of a single covering machine. The difficulty arises because the spindles need to be rotated at a rate corresponding to the number of revolutions suited to each kind of yarn and because there are differences in quality between core yarns and covering yarns. The number of twisting operations required for each kind of yarn differs when the covering operation is performed. The number of revolutions of all spindles in the conventional spindle driving device is, however, arranged to be the same and thus, it is difficult to produce various kinds of covering yarns through use of a single unit of a covering machine.
Also, the endless belt is so long in length that it is difficult to press the belt in the manner necessary to contact each one of the spindles with uniform contact pressure along the entire circumference of the endless belt. Besides, some rotational differences occur among the spindles as a result of the differences in tensile force between the tensioned side and the relaxed side of the endless belt. Accordingly, there occur some differences in the number of twistings of the covering yarns which result from the aforementioned differences in spindles, thereby causing variations in uniformity with respect to the quality of the yarns.